The art of ex vivo culture of hematopoietic stem cells and their progeny has advanced considerably in recent years. Notably, the identification of stage-specific antigens on the surfaces of cells and of a variety of extrinsically acting cytokine cocktails that drive expansion of cell numbers, as well as the development of biological assays to detect the potency of various end products (i.e., cell types) of expansion have provided the fundamental tools and knowledge to explore more refined methods for the controlled modulation of the proliferation and differentiation of cells in culture. While the present state of the art in hematopoietic cultures allows for the expansion of large numbers of cells, such cultures can lead to the exhaustion of the stem cell and progenitor cell pools.